"Yo, check me out, I'm so f-ing big & bad I don't HAVE to wait for my turn on the leg press! Take a hike, dweeb!" No matter how bad you think you are, I'm sure you wouldn't appreciate being pushed around by Greg Kovacs or some other 350+ dude yourself. Size is NOT an excuse to behave like an ass, bluntly put.
In fact, if you're big, that usually indicates that you've spent a number of years in the gym and therefore should have acquired a certain amount of gym manners. So what in fact you WOULD be proving is that you're not only insecure, you're also making an effort in fulfilling the myth of the muscle head-with-an-attitude. Great going, dude.
If you've finished a set of leg curls and discover that you've left a nice little ocean on the cushion big enough for your nephews to play sea battle on it, it might be a good idea to wipe it off. Think about it - what is sweat? Essentially the same as urine. Do you feel like lying down and rubbing your face in another person's urine? Of course, some wise guy might argue that it evaporates and therefore is no problem.
Except for the slight detail of it being the WATER evaporating. The really yucky part remains, and is ready for the next person who, unknowingly, wets it with his own sweat and gets it all, so to speak. Think of it as "instant urine" next time you see a slob walk away from his pool of sweat with that excuse.
Fortunately, most people know what the word "deodorant" means. It's also commonly found as part of standard gear in gym bag, which is even better. However, it does not replace the shower. There's a major difference between being clean/hiding stench. It also goes for gym clothes, which should be washed often. Shoes can be quite something else in this category, as they have this tendency to turn into virtual stink bombs.
Preferably, alternate several pairs and take out the inner soles when not in use. Avoid storing them where air circulation is bad, and storing them in the gym bag 24 hours a day is banned! There's also the aspect of certain foods to avoid before going the gym. If you like stews with garlic by the pound, go ahead and indulge - but not the night before going to the gym! Other foods, such as pigging out on beans before doing squats, is not recommended either. For obvious reasons.
Keep the weights where they belong - NOT on the floor, on the machines or on the barbell! When you leave, make sure to have returned ALL the plates, and more specifically, to their designated spots. The heaviest plates usually go low while going lighter as you go up, and even though a 100 lb-plate isn't a problem for you when standing on your tiptoes, it might be slightly more difficult for 61-year old, 5'3" aunt Marge to reach the 10-lb plates which lies hidden behind your 100-lb'er.
Of course, kicking over someone's water bottle is bad but even if it's your own, you should wipe it up. Do NOT wait for someone in the staff - just do it! You kicked it over, didn't you? Well, then YOU take care of it!
If you see someone, beginner or not, who does an exercise not only wrong but directly HARMFUL to them, wait until their set is done and gently approach them about the problem. That doesn't mean:
"Yo, bozo, you're all WRONG!" but more like: "Have you tried keeping your back straighter? Perhaps you might want to try it in your next set, as it can help you target the muscles better and decrease the risk of injury?" Keep it low profile, without embarrassing the other one. Gentle inquiry and correction is better than wiseass-guru attitude. If the person appear to take it the wrong way, it's their problem so just go back to doing your thing. He'll wise up on his way to the emergency room.
Needless to say, it works both ways - if YOU end up being the one corrected, swallow the pride for a minute and embrace the chance of having your form improved!
Of course there are many more, minor rules, such as not slamming down the weights, refusing to share a machine, loud cursing and so on, but these are the most basic rules. In my humble opinion. If you've grasped those 5 basic rules, I say the other rules is pretty much mere extension of the basics.